Arrangement for controlling edge diffraction of microwaves



Oct. l, 1957 M. .LHRLICH Erm. 2,808,586

ARRANGEMENT Foa coNTRoLLrNG EDGE DIFFRACTIQN oF mcRowAvEs Filed NOV. 27, 1953 WNW. Sn. WNN.

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ARRANGEMENT FOR CONTROLLING EDGE DIFFRACTEN F MICROWAVES Morris J. Ehrlich, Venice, and William G. Sterns, Los Angeles, Calif., assignors, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application November 27, 1953, Serial No. 394,806

Claims. (Cl. 343-833) This invention relates generally to electromagnetic wave transmission and reception and more particularly to the control of edge diffraction in microwave antennas.

`In certain applications of microwave antennas it is often necessary to minimize the amount of energy radiated in some directions as well as maximize the amount of energy radiated in other directions. In terms of antenna radiation patterns, this requirement generally involves the provision of a principal lobe having a specified width, and the reduction of subsidiary or side lobes to a particular level relative to the principal lobe. Since the presence of side lobes within angular regions remote from the principal lobe is often of little consequence, in effect, the problem is one of forming a main beam without producing side lobes of too great an amplitude within certain other angular regions.

As is well known, increasingly narrow beams may be formed by increasing the antenna aperture, so that in the usual case, the formation of a main beam having a particular width is readily accomplished. When the antenna aperture is to be used efficiently, however, so as to form the narrowest beam possible with a given size aperture, considerable difllcuty has been encountered with respect to side lobes. Very often this difficulty arises from the effects of edge diffraction in vthe antenna system.

One conventional type of antenna which serves to illustrate the deleterious effects of edge diffraction is a parabolic reflector antenna including a radiating element in combination with a primary reflector such as a disk. The radiating element and primary reflector combination, generally referred to as the feed, simulate a point source located near the parabolic focus and adapted to illuminate the parabolic or secondary reflector. 'Since the primary reector must be small in terms of operating wavelengths so as not to obstruct the principal beam formed by the parabolic reflector, all of the energy from the feed does not radiate in the direction of the parabolic reflector. That is to say, whereas most of the energy from the feed is directed towards the parabolic reflector, some of the energy is directed more or less at right angles to the parabolic axis, and some of the energy is diffracted by the edge of the primary reflector. Because of this edge diffraction, there is formed a diffraction beam axially aligned with the principal `beam for-med by the parabolic reflector, but disassociated therefrom with respect to phase relations. Accordingly, the diffraction beam interferes with the principal beam causing serious perturbations in the antenna radiation .pattern over a relatively wide angular region. Although this effect may be alleviated at a single frequency by choosing a focal distance which is most satisfactory considered from the standpoint of phase relations between the principal beam and the diffraction beam, little can be accomplished in this way when the antenna system is to be operated over a 'band of frequencies.

In the past, another approach to the problem has been to taper the illumination of the primary reflector and thereby decrease the amount of energy in the diffraction nited States Patent rice beam. Accordingly, energy from the radiating element is concentrated on the central portion of the primary reflector and gradually decreased in intensity towards the outer portions. When this type of illumination is used, however, the energy from the feed is less widely distributed with the result that the outer portions of the parabolic reflector are illuminated less strongly. As a result, the principal beam formed `by the parabolic reflector becomes relatively broad. If then, the size of the primary reflector were to be increased in order to better illuminate the parabolic reflector, a `greater obstruction would be presented to the principal beam. Consequently, the principal beam would tend to be deformed and adjacent side lobes increased partly because of the diffraction of the principal beam itself 'by the primary reflector.

Accordingly, this invention discloses an arrangement Ifor controlling edge diffraction effects over a relatively broad band of frequencies in order to minimize perturbations or side lobes within certain angular regions. In the case of the aforementioned parabolic reflector antenna, there is provided a plurality of radiators comprising apertures positioned close to the edge of the primary reflector. If a disk-type primary reflector is utilized, the apertures are positioned along the edge of the disk vso as to provide a ring-shaped source. This ring source is adapted to reradiate a portion of the energy from the feed radiating element in a manner to compensate for the diffraction effects produced by the edge of the disk. In other words, the waves emanating from the ring source are larranged to destructively interfere with the diffracted waves in the angular region of interest. To accomplish this result, the amplitude of the waves from the ring source may be controlled by choosing apertures of proper size, and the phase of these waves may be adjusted by placing dielectric material of suitable thickness over the apertures. Since the ring source is positioned close to the diffraction source, namely the edge of the disk, the arrangement of this invention is relatively insensitive to frequency variations and therefore particularly well adapted for broad-band operation.

It is an object of this invention, therefore, to provide a method of controlling the diffraction of microwaves 'by a conductive body.

It is another object of this invention to provide means for controlling the direction of propagation of diffracted electromagnetic waves.

It is still another object to provide sources of radiant energy adapted to destructively interfere with energy diffracted by the edge of an antenna reflector.

It is a further object to provide means for substantially reducing the undesirable effects of edge diffraction in .antenna systems operating over a broad-'band of frequencies.

The novel features of this invention, together with further objects and advantages thereof, will be better understood when considered in connection with the accompanying drawings in which:

Fig. l is a view in elevation of a feed structure comprising a waveguide-type radiating element and a reflecting disk provided with an arrangement of apertures in accordance with this invention;

Fig. 2 is a family of curves indicating the front-toback ratio of the radiating element and reflecting disk combination of Fig. l as a function of dielectric thickness;

Fig. 3 is a perspective view of a parabolic reflector antenna including a dipole-type radiating element and a reflecting disk provided with apertures in an arrangement according to this invention;

Fig. 4 is a parabolic reflector antenna including a horntype radiating element provided with an arrangement of apertures in accordance with this invention; and

Fig. 5 is a cylindrical reflector antenna including a slot-type feed structure provided with an arrangement of apertures according to this invention.

Fig. l illustrates a feed structure comprising a length of round uniconductor waveguide 11 and a reflecting disk 12 affixed thereto by means of a dielectric support member 13. In particular, waveguide 11 has a diameter of approximately 0.7 wavelength in free space and is therefore adapted to transmit electromagnetic waves in the TEn Inode. At one end thereof, a flange 1dis provided for attachment to support member 13 which has a mating flange portion 1S as shown in Fig. l. Cornpleting support member 13 is a stem portion 16 fastened to the center of disk 12 so that in effect waveguide 11, support member 13, and disk 12 are aligned coaxially.

Disk 12 is provided with a ring of twenty four holes 17 disposed about its center and in proximity to its edge. Overlying holes 17 on the side of disk 12 away from waveguide 11 is a ring of dielectric material 18, having a dielectric constant, E, of approximately eight. It has also been found desirable to form a rim 19 at the edge of disk 12, rim 19 being at right angles to the surface of disk 12 and overlying the outer edge of dielectric ring 18.

The operation of the feed structure of Fig. l may be best described with reference to Fig. 2 wherein the frontto-back ratio of the feed structure as a function of dielectric ring thickness is graphically represented for four different diameters of holes. The term front-to-back ratio refers to the relative magnitudes of the axial disposed frontal beam or lobe comprising energy reflected by the surface of disk 12, and the axially disposed backward lobe comprising energy diffracted by the edge of disk 12. As seen from Fig. 2, the provision of holes 17, in the arrangement of this invention, greatly increases the front-to-back ratio particularly when the size of holes 17 and the thickness of dielectric ring 18 is optimum. This is because holes 17 in combination with dielectric ring 1S serve as additional sources of electromagnetic energy which interferes with the diffracted energy. By optimizing the size of holes 17, and the thickness of dielectric ring 18, the amplitude and phase of the reradiated energy is so adjusted that destructive interference takes place with the energy comprising the backward diffraction lobe, thereby increasing the front-to-back ratio. The amplitude of the reradiated energy is proportional to the square of the hole diameter, and the phase of the energy is determined from the well known formula la=\/` ld, where la is the path lenfth of electromagnetic waves in air equivalent to the path length ld in a dielectric medium of dielectric constant e. ln this regard it is apparent that a further change in the magnitude of the reradiated energy may be obtained by varying the number of holes utilized.

Fig. 3 illustrates the application of this invention to a parabolic reflector antenna including a parabolic reflector 31, having in the vicinity of its focus, a dipoletype radiating element 32 and a disc-type primary reflector 33. Dipole element 32 is supplied with electromagnetic energy from a length of coaxial transmission line 34 disposed along the axis of the parabolic or secondary reflector 31. As is well known, energy radiated by dipole element 32 is in part reflected by primary reflector 33 so as to form a beam or lobe directed towards parabolic reflector 31. Parabolic reflector 31 in turn reflects this energy again in a manner to form a second or principal beam of relatively narrow width, and having its axis coextensive with the axis of the parabolic reflector. Some of the energy from dipole element 32 is diifracted by the edge of primary reflector 33, however, which would ordinarily result in the formation of a diffraction beam radiating in the same direction as the beam produced by parabolic reflector 31. Because of interference between the diffraction beam and the principal beam, perturbations in the radiation pattern would then appear within a relatively large angular region about the parabolic axis.

According to this invention, therefore, there is provided a plurality of apertures comprising holes 37, formed in a ring about the center of primary reflector 33 in proximity to its edge. Overlying holes 37 there is also provided a ring of dielectric material 38. Holes 37 in combination with dielectric ring 38 serve as sources of electromagnetic energy having such an amplitude and phase as to destructively interfere with the energy comprising the diffraction beam. Thus, the diffraction beam is substantially reduced, thereby eliminating to a large extent its undesirable effects on the principal beam and the radiation pattern of the antenna generally.

Fig. 4 illustrates another application of this invention wherein the antenna utilized comprises a parabolic reflector 4l, which is illuminated by a horn-type radiating element 42 having a flanged portion 43. Horn-type radiating element 42 is located near the focus of parabolic reflector 4l and supplied with electromagnetic energy from a length of waveguide 44. Since a portion of the energy radiated by element 42 is diffracted by the edge of flanged portion 43 so as to form a diffraction beam which interferes with the principal beam formed in the manner previously described, there is provided a ring of holes 47 in flanged portion 43. For the purpose of phase adjustment, a ring of dielectric material 48 overlying holes 47 is also included. The radiation pattern of the antenna is substantially improved owing to the reduction of the diffraction beam and its undesirable effects on the radiation pattern of the antenna, particularly within an angular region about the axis of parabolic reflector 41.

Fig. 5 illustrates the application of this invention to still another type of antenna utilized in height-finding systems and comprising a feed structure 51 and cylindrical reflector 52. Feed structure 51 includes a length of waveguide 53 supplying electromagnetic energy to an array of slot-type radiating elements 54. Radiant energy from slot elements S4 is then directed towards reflector 52 by means of a pair of vanes 55 disposed at an acute angle to a vertical axis 56. Reflector 52, in turn, is adapted to provide a radiation pattern which, on the average, varies in amplitude as cosec2 0, where 0 represents angular deviation with respect to axis 56.

Partly because of diffraction effects produced by the edge of vanes 55, appreciable perturbations or fluctuations in the radiation pattern generally occur at small angles 0. To minimize these fluctuations, there is provided a linear array of radiating elements comprising holes 57 positioned along the edge of each vane 55, and strips of dielectric material 58 overlying each of these arrays of holes. By utilizing dielectric strips S8 of suitable thickness and making holes 57 of proper size, the energy radiated by the holes may be so adjusted in amplitude and phase as to destructively interfere with the diffracted energy within an angular region close to axis 56. In this way the perturbations or fluctuations in the radiation pattern may be substantially reduced.

On the other hand, if it is desired to eliminate perturbations at relatively large angles in terms of 6, a linear array of holes would be provided along the edges of reflector 52. This is because the edge of reflector 52 gives rise to diffracted energy causing destructive interference at these large angles. As is apparent therefore, an arrangement of apertures in accordance with this invention will be equally applicable to other types of antennas wherein the effects of edge diraction prove undesirable.

Similarly, other types of apertures such as slots may be more advantageous than holes in some cases.

What is claimed is:

l. A microwave system including a reflector for intercepting radiant microwave energy, said reflector being provided with a plurality of apertures for reradiating a part of energy intercepted by said reflector to combine with energy diffracted by the edge of said reflector, and dielectric material covering said apertures for retarding the phase of said reradiated energy to cause said reradiated energy to destructively interfere with said diiracted energy.

2. A system according to claim 1 wherein said apertures are substantially circular, and have a diameter in the range between 0.1 and 0.7 of a wavelength of said radiant energy.

3. A system according to claim 1 wherein said apertures are adjacent the difracting edge of said reflector.

4. A microwave antenna system including a radiating element and a reflector for intercepting energy radiated by said element and directing part of said intercepted energy into a selected angular region of space, said reflector including means for reradiating another part of said intercepted energy into a second angular region of space to combine in said second region with energy diffracted by the edge of said rellector, and means for retarding the phase of said reradiated energy to produce destructive interference between said reradiated energy and said diffracted energy.

5. A microwave antenna system including a radiating element and a reflector for intercepting energy radiated by said element and directing part of said intercepted energy into a selected angular region of space, said reflector being provided with a plurality of apertures for transmitting another part of said intercepted energy into a second region of space wherein energy difracted by the edge of said reflector is propagated, and dielectric material covering said apertures for retarding the phase of said reradiated energy to effect destructive interference between said reradiated energy and said dilfracted energy in said second region.

6. A system according to claim 5 wherein the transverse dimensions of said apertures are in the range between 0.1 and 0.7 of a wavelength of said wave energy.

7. A system according to claim 6 wherein said apertures are positioned along the edge of said reflector.

8. A microwave antenna system comprising a radiating element, and a reflecting disk having a rim disposed at right angles to the surface of said disk, said disk being provided with a ring of circular apertures disposed about the center of said disk in proximity to its edge, and a layer of dielectric material covering said apertures.

9. A microwave antenna system comprising a source of radiant microwave energy; a principal reector; an auxiliary reflector disposed adjacent said source for directing microwave energy radiated by said source towards said principal reector, said principal reflector forming said microwave energy into a narrow beam; said auxiliary reector being provided with a plurality of apertures for reradiating a part of the microwave energy radiated by said source substantially along the axis of said beam, and dielectric material covering said apertures for retarding the phase of said reradiated energy to produce destructive interference between said reradiated energy and wave energy diifracted by the edge of said auxiliary reflector.

10. A microwave antenna system according to claim 9 wherein said apertures are circular and have a diameter in the range between 0.1 and 0.7 of a wavelength of said radiant microwave energy.

References Cited in the file of this patent UNITED STATES PATENTS 2,460,869 Braden Feb. 8, 1949 2,556,087 Iams June 5, 1951 FOREIGN PATENTS 595,653 Great Britain Dec. 11, 1947 

